This invention relates to photocathodes and more particularly to a method for forming a photocathode which exhibits improved high temperature operating characteristics.
A previous type of photoemitting surface is a semitransparent multialkali photocathode such as described in U.S. Pat. Nos. 2,770,561 to A. H. Sommer and U.S. Pat. No. 3,372,967 to F. R. Hughes. Generally, photocathodes of this type which have been sensitized with cesium (cesiated photocathodes) have substantially higher sensitivities of response than noncesiated photocathodes. However, such cesiated photocathodes have been found inadequate for certain applications. For example, photomultiplier tubes having cesiated photocathodes have been used for scintillation counting, in applications, such as, for example, geophysical exploration in which the ambient temperature of operation approaches 150.degree. C. At such temperatures, the cesiated photocathode appears to decompose and the expected useful life of the device is severely restricted. Moreover, high temperature operation in general, above 85.degree. C., of conventional photomultiplier tubes with noncesiated photocathodes, tends to make the tube extremely sensitive to higher operational voltages, which, if applied to the device, are known to cause spurious scintillation counts and general instability of the processed signal, due to regenerative effects within the tube. An improved noncesiated photocathode comprising potassium, sodium and antimony is described in U.S. Pat. No. 3,828,304 to McDonie. The McDonie bialkali photocathode operates satisfactorily to ambient temperatures of about 175.degree. C.; however, the photocathode appears to decompose as the temperature approaches 200.degree. C. Present geophysical exploration requirements demand a stable photocathode that will survive ambient temperatures of about 200.degree. C.